Flow into area
The conversion is the standard orifice relationship. Air accelerating through a hole under a pressure difference reaches a velocity of the square root of twice the pressure divided by the density, and the flow is that velocity times the area times a discharge coefficient that accounts for the hole not being an ideal nozzle.
At 50 pascals and a density of 0.075 pounds per cubic foot, the velocity works out at 9.12 metres per second. A 4,000 CFM reading is 1.8878 cubic metres per second, and at a discharge coefficient of 0.61 the area comes to 0.3392 square metres, which is 525.8 square inches — a square 22.9 inches on a side. In a 16,000 cubic foot house that same reading is 15.0 air changes an hour at 50 pascals.
Two things about that number are worth being blunt about. The discharge coefficient is an assumption and the area is inversely proportional to it, so a coefficient of 0.5 instead of 0.61 gives 641 square inches instead of 526. And the area is stated at the test pressure. Flow goes roughly as the square root of pressure, so on a still day at two or three pascals the same shell passes a small fraction of the fan figure.
The comparison is the point
An equivalent hole on its own is a party trick. What makes it useful is holding it next to a list of gaps you measured with a ruler, because the difference tells you how much of the house you have not found.
| Site | Gap | Run | Area |
|---|---|---|---|
| Window and door perimeters | 1/32 in | 240 ft | 90.0 sq in |
| Bottom plate to subfloor | 1/32 in | 180 ft | 67.5 sq in |
| Recessed light housings | 3/16 in | 24 ft | 54.0 sq in |
| Dryer and bath vent penetrations | 1/4 in | 6 ft | 18.0 sq in |
| Attic hatch perimeter | 1/8 in | 11 ft | 16.5 sq in |
| Total found | 246.0 sq in |
The window and door perimeters at a thirty-second of an inch beat the vent penetrations at a quarter inch by five to one, because they run for 240 feet and the vents run for 6. Area is width times length, and length is the term people underestimate. This is why sealing the plate lines and the perimeters earns more than chasing the four places that whistle, and why the gaps that get noticed are rarely the gaps that matter.
Against the 525.8 square inches the fan implies, that 246 is 47 percent. Slightly under half of the leakage located, from a careful pass through the finished rooms, is a typical first result — and the missing half is the interesting half.
Where the missing area usually is
On a first pass most people find well under half the effective area. What is missing is almost never around the visible openings. It is in the places where the conditioned space touches the attic, the crawl space or an attached garage: a dropped soffit over kitchen cabinets that is open to the attic along its whole length, a chase around a flue or a plumbing stack, a balloon-framed wall cavity open at the top plate, the back of a knee wall, and the top plates themselves. Those are found by going into the attic and looking, not by measuring gaps in the finished rooms.
Once the sites are found, the material take-offs are elsewhere: foam by the can or the kit for the static gaps, tubes of sealant for the beads, weatherstrip and sweeps for the joints that move, and an insulated cover for the hatch. When there is a before and an after reading, the air sealing payback calculator turns the difference into fuel and money using your own degree days and price.
What this page will not tell you
It will not say whether a reading is good. Air-tightness targets belong to specific programs and specific code editions, they differ by jurisdiction, they change, and they come bundled with ventilation requirements that are part of the same package. Handing out a threshold without the ventilation half of it would be worse than useless. It also will not tell you how tight is too tight, because that depends entirely on what burns fuel inside the house and how it gets its air — which is a measurement with a combustion analyser, not an arithmetic problem.
Questions people ask
How do you convert CFM50 into a hole size?
Through the orifice equation. The velocity through a hole is the square root of twice the pressure difference divided by the air density, and the flow is that velocity times the area times a discharge coefficient. At 50 pascals and 0.075 pounds per cubic foot the velocity is 9.12 metres per second, so 4,000 CFM at a coefficient of 0.61 works out to about 526 square inches, a square roughly 22.9 inches on a side. The coefficient is the soft part: the answer moves in inverse proportion to it, so 0.5 instead of 0.61 gives 641 square inches from the same reading.
What discharge coefficient should I use?
There is no single correct value, which is why it is an input here rather than a constant. A thin sharp-edged orifice sits near 0.6, a rounded entry or a long smooth passage is higher, and a rough narrow crack with obstructions along it is lower. Different conventions in the field use different values, so a leakage area quoted without the coefficient it assumed is not comparable with another one. If you are comparing your own before and after, the coefficient cancels out and the choice barely matters.
Why does my measured gap list add up to more than the fan says?
Because a geometric gap is not an open hole. A crack between two framing members is long, narrow and rough, partly blocked along its length, and it passes far less air than a clean orifice of the same area. It is also easy to overstate a gap width by eye. If the ruler total comes out well above the fan figure, remeasure the biggest contributor first and check whether it is genuinely open all the way through the assembly or just open on the face you can see.
Is a smaller number always better?
A tighter shell uses less energy and is more comfortable, but tightness and ventilation are one decision rather than two. Air that used to arrive through gaps also fed combustion appliances and carried moisture out, so past a certain point a house needs outdoor air brought in deliberately and needs anything that burns fuel checked under worst-case conditions. Where that point is depends on the house, the appliances and the climate. This page gives you an area and deliberately gives you no verdict on it.
Can I estimate the equivalent area without a blower door?
You can build the gap list half of this page from measurements and get a geometric area, and that is genuinely useful for ranking sites against each other. What you cannot get without a fan is the total, because the sites that dominate most houses are the ones you cannot see from inside the finished rooms. The comparison between the two halves is the whole value here, and it needs the fan reading to exist.